This application is based on and claims under 35 U. S. C. xc2xa7119 with respect to Japanese Patent Application No. 2000-137694 filed on May 10, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to variable valve timing systems. More particularly, the present invention pertains to a variable valve timing system for controlling the opening and closing time of an intake valve and an exhaust valve of a vehicle engine.
Known variable valve timing system is described in Japanese Patent Laid-Open Publication No. H09-264110. The disclosed variable valve timing system includes a housing member disposed in the driving force transmitting system for transmitting the driving force from a crankshaft of a combustion engine to a camshaft for controlling the opening and closing of either an intake valve or an exhaust valve of the combustion engine. The housing member rotates in one unit with either the crankshaft or the camshaft. The variable valve timing system further includes a rotor member rotatably mounted to a shoe portion provided on the housing member. The rotor member forms an advanced angle chamber and a retarded angle chamber at a vane portion in the housing member and integrally rotates with either the camshaft or the crankshaft. The aforementioned known variable valve timing system further includes a torsion spring for rotatably biasing the rotor member relative to the housing member, a stopper mechanism for defining the initial phase of the housing member and the rotor member, a lock mechanism for restricting relative rotation between the housing member and the rotor member at the initial phase, and a hydraulic pressure circuit for controlling supply and discharge of the operation fluid for the advanced angle chamber and the retarded angle chamber as well as for controlling supply and discharge of the operation fluid from the lock mechanism.
With respect to the variable valve timing system disclosed in the prior art, the hydraulic pressure control condition of the hydraulic pressure circuit is promptly switched from the initial hydraulic pressure control condition in which the rotor is maintained at the initial phase so as to lock the relative rotation between the housing member and the rotor member by the lock mechanism a condition in which the lock mechanism is released so as to shift the phase to the target advanced angle value. In the foregoing structure, before the lock mechanism is released by the operation fluid supplied from the hydraulic pressure circuit, the retract movement of the lock mechanism from the locked position to the unlocked position may be disturbed due to the large sliding resistance of a lock member, such as a lock pin, of the lock mechanism which is caught between the rotor member and the housing member accompanying to the relative rotation therebetween by the rotational force of the torsion spring. The lock pin restricts relative rotation between the rotor member and the housing member by engaging with both of them at a locked position and allows relative rotation of the rotor member and the housing member by retracting one of them at the unlocked position.
In light of the foregoing, the present invention provides a variable valve timing system for advancing and retarding valve timing of intake and exhaust valves of a combustion engine. The variable valve timing system is programmed to control a hydraulic pressure control condition of a hydraulic pressure circuit in the system. The hydraulic pressure control condition is shifted from an initial hydraulic pressure control condition in which a rotor is maintained at an initial phase and locked by a lock mechanism, to a phase shiftable hydraulic pressure control condition in which a volume of either an advanced or retarded angle chamber is varied to reach a target angle via a transitional hydraulic pressure condition in which the rotor is maintained at the initial phase and the lock mechanism is released.